Automatic Transmissions and Transaxles, 4th Edition Page 203 (203 of 495)

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Chapter 9 Automatic Transmission Control Components 203
Oil filter
Transmission
solenoid
Pressure switch
B
A
Pressure control (PC)
solenoid valve
Torque
converter
clutch (TCC)
solenoid valve
Torque converter
clutch pulse width
modulation
(TCC PWM)
solenoid valve
3-2 shift solenoid (SS)
valve assembly
Automatic transmission
fluid pressure (TFP)
manual valve position switch
1-2 Shift
solenoid (SS)
valve
2-3 Shift
solenoid (SS)
valve
Figure 9-27. Most of the control solenoids in a modern transmission are installed on the valve body, inside the oil pan. This transmis-
sion has six solenoids and a pressure and temperature switch assembly. (General Motors)
Figure 9-26. A—A typical pressure switch used on late-model
transmissions. Note the two connections for current to flow in
and out of the switch. Some switches have only one connection.
B—A pressure switch installed on the transmission valve body.
(General Motors)
Solenoids
Electric solenoids control the operation of some trans-
mission hydraulic components. Solenoids on modern trans-
missions are installed on the valve body. These are con-
nected to the vehicle through a wiring harness having an
electrical connector that is mounted on the case. See
Figure 9-27. On a few older transmissions, the solenoids are
threaded directly into the transmission case as shown in
Figure 9-28.
A sectional view of a typical solenoid is shown in
Figure 9-29. When energized, the solenoid valve functions
by opening a small, self-contained valve or ball. This action
causes transmission fluid in the connecting passageway to
be exhausted. Loss of pressure in the passageway causes
another valve to move, and action from this point is com-
pletely hydraulic.
An example of a transmission solenoid is the detent
solenoid used on some automatic transmissions. This sole-
noid is also called the kickdown solenoid or the downshift
solenoid. The detent solenoid is energized by closing the
kickdown switch, which is located near the accelerator.
Figure 9-30 is a simplified diagram of a detent solenoid
circuit.
Another common solenoid is the lockup solenoid,
which is used to engage the torque converter lockup clutch,
Figure 9-31. When the solenoid is energized, its integral

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Copyright by Goodheart-Willcox Co., Inc.
Chapter 9 Automatic Transmission Control Components 203
Oil filter
Transmission
solenoid
Pressure switch
B
A
Pressure control (PC)
solenoid valve
Torque
converter
clutch (TCC)
solenoid valve
Torque converter
clutch pulse width
modulation
(TCC PWM)
solenoid valve
3-2 shift solenoid (SS)
valve assembly
Automatic transmission
fluid pressure (TFP)
manual valve position switch
1-2 Shift
solenoid (SS)
valve
2-3 Shift
solenoid (SS)
valve
Figure 9-27. Most of the control solenoids in a modern transmission are installed on the valve body, inside the oil pan. This transmis-
sion has six solenoids and a pressure and temperature switch assembly. (General Motors)
Figure 9-26. A—A typical pressure switch used on late-model
transmissions. Note the two connections for current to flow in
and out of the switch. Some switches have only one connection.
B—A pressure switch installed on the transmission valve body.
(General Motors)
Solenoids
Electric solenoids control the operation of some trans-
mission hydraulic components. Solenoids on modern trans-
missions are installed on the valve body. These are con-
nected to the vehicle through a wiring harness having an
electrical connector that is mounted on the case. See
Figure 9-27. On a few older transmissions, the solenoids are
threaded directly into the transmission case as shown in
Figure 9-28.
A sectional view of a typical solenoid is shown in
Figure 9-29. When energized, the solenoid valve functions
by opening a small, self-contained valve or ball. This action
causes transmission fluid in the connecting passageway to
be exhausted. Loss of pressure in the passageway causes
another valve to move, and action from this point is com-
pletely hydraulic.
An example of a transmission solenoid is the detent
solenoid used on some automatic transmissions. This sole-
noid is also called the kickdown solenoid or the downshift
solenoid. The detent solenoid is energized by closing the
kickdown switch, which is located near the accelerator.
Figure 9-30 is a simplified diagram of a detent solenoid
circuit.
Another common solenoid is the lockup solenoid,
which is used to engage the torque converter lockup clutch,
Figure 9-31. When the solenoid is energized, its integral

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202 Automatic Transmissions and Transaxles
Ignition key
Key cylinder
Camshaft
Key lock solenoid
Lock
pin
Figure 9-23. This solenoid is installed on the ignition key assembly and is operated by a small computer. It prevents accidental starts
in gear and reduces the chance of vehicle theft. (Lexus)
Selector
lever button
Grooved pin
Shift lock
override button
Shift lock
computer
Shift lock solenoid
Plate stopper
Shift lock
override link
Shift lock
control
switch
Shift lock
plate
Figure 9-24. This shift-brake interlock uses a small computer
and a solenoid to prevent damage to the drive train and engine.
The shift selector lever is connected to a small computer. The
computer triggers the solenoid, which will not allow the lever to
be moved out of gear unless the brake is applied. (Lexus)
Computer
Oil passage
Block
Electrical connection
Electrical
contacts
Diaphragm
Figure 9-25. The typical hydraulic pressure sensor is a switch
that tells the computer that a hydraulic circuit is pressurized. Oil
pressure acts on the switch diaphragm to open or close the
electrical contacts.
Electrical Components
Modern transmission hydraulic systems have electrical
and electronic components. On newer vehicles, electrical
and electronic components control all transmission func-
tions, including gear changes. Some of the more common
components are discussed below. These components and
the devices that control them will be discussed in more
detail in Chapter 12.
Switches
Switches use electrical contacts to energize or de-
energize a solenoid or motor, or to send an electrical signal
to another electrical device. Switches generally have two
contacts. When the contacts are touching, the switch is in
the on position and current can flow through the switch.
When the contacts are separated, the switch is in the off
position and current cannot flow. Some pressure switches
have one contact, and current flow is grounded through the
switch body in the on position.
The driver activates some switches through shift link-
age connected to the gear selector lever. Another type of
driver-operated switch is installed on the accelerator linkage
to operate the detent (passing gear) valve system. These types
of switches are discussed in more detail later in this chapter.
Another commonly used switch is the pressure switch.
Modern transmissions usually contain several pressure
switches. Pressure switches are opened and closed by
transmission hydraulic pressures, Figure 9-25. Figure 9-26
illustrates a typical pressure switch and its general location
on the valve body.

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204 Automatic Transmissions and Transaxles
Converter lockup
solenoid Overdrive cancel
Vacuum
modulato
Downshi
solenoid
Figure 9-28. Some older transmissions have solenoids installed
on the outside of the case. (Nissan)
Coil
winding
Return
spring
Drain Drain
Hydraulic
valve plunger
Line pressure
Wire
terminal
Figure 9-29. A typical solenoid consists of a coil winding, a
plunger, and a return spring. The winding, when energized, pulls
the plunger against return spring pressure. This causes trans-
mission fluid to be exhausted and activates the particular con-
trol circuit. (Chrysler)
Kickdown switch
Downshift solenoid
Accelerator pedal
Figure 9-30. Simplified diagram of a typical detent solenoid
circuit is shown here. Pressing the accelerator to the floor
closes contacts on the kickdown switch and energizes the
solenoid. The valve opens and fluid is exhausted, reducing
hydraulic circuit pressure. (Nissan)
Lockup solenoid
Line pressure
(from 1-2 shift valve)
Converter pressure
Drain
Line pressure
(from pump)
To torque converter
(lockup on)
To oil
cooler
To torque
converter
(lockup off)
Lockup
relay
valve
Figure 9-31. The lockup solenoid operates the hydraulic
portion of the torque converter lockup relay valve on many mod-
ern transmissions. (Chrysler)
valve is unseated, causing transmission fluid in the connect-
ing line to be exhausted. Loss of this fluid reduces circuit
pressure. This causes the lockup relay valve to move in such
a way as to apply the clutch. The first lockup solenoids
were often operated by vacuum and pressure switches.
Today, most lockup solenoids are operated by the on-board
computer.
Summary
Transmission oil pumps use gears, rotors, or vanes that
cause the hydraulic system to become pressurized. The oil
pump is usually driven by lugs on the rear of the torque
converter. Some older transmissions have a second oil
pump that is driven by the transmission output shaft.
Transmission oil filters are used to remove particles
from the automatic transmission fluid. Fluid being drawn
into the oil pump must first pass through the filter. This
keeps fluid contaminants from entering the pump or the
other hydraulic components.
The transmission oil cooler removes heat from trans-
mission fluid. Fluid that has been overheated does not
lubricate properly. Further, it breaks down into sludge and
varnish, which plug passageways. The oil cooler is located
in the engine cooling-system radiator.
The valve body contains the manual valve, shift valves,
and other hydraulic system valves. Valve bodies usually

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